Subnetting in 293

Discussion in 'MCSE' started by RogueIT, Jun 12, 2009.

  1. RogueIT

    RogueIT Guest

    In the MS Press 70-293 book they have a question about subnetting.
    In the question they ask what is the first subnet of 10.0.0.0/19.
    I thought it would be 10.0.0.0 with the first usable address as 10.0.0.1,
    but they give the answer as 10.0.32.1 to 10.0.63.254.
    Why isn't 10.0.0.1 to 10.0.31.254 the first subnet?

    Thanks,
    RogueIT
    RogueIT, Jun 12, 2009
    #1
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  2. RogueIT

    JohnB Guest

    The IP address they give for that question is a poor example.

    That is a Class A address, which has an 8 bit network mask.
    With the example using 19 bits, having borrowed 11 bits from the host
    portion of the address.
    So the mask in dotted decimal format is 255.255.224.0
    Which gives a subnet block size of 32 (256-224=32)
    So the address ranges for each subnet would be:
    10.0.0.0 - 10.0.31.255
    10.0.32.0 - 10.0.63.255
    10.0.64.0 - 10.0.95.255
    10.0.96.0 - 10.0.127.255
    and so on, up to....
    10.0.224.0 10.0.255.255

    With the first address being the subnet, and the last being the broadcast
    address, for each range
    So you're right, the *first* subnet is 10.0.0.0
    The answer they give is the usable *range*, which is different than a subnet
    number. And their range, is the second range.






    "RogueIT" <> wrote in message
    news:...
    > In the MS Press 70-293 book they have a question about subnetting.
    > In the question they ask what is the first subnet of 10.0.0.0/19.
    > I thought it would be 10.0.0.0 with the first usable address as 10.0.0.1,
    > but they give the answer as 10.0.32.1 to 10.0.63.254.
    > Why isn't 10.0.0.1 to 10.0.31.254 the first subnet?
    >
    > Thanks,
    > RogueIT
    >
    JohnB, Jun 12, 2009
    #2
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  3. "RogueIT" <> wrote in message
    news:...
    > In the MS Press 70-293 book they have a question about subnetting.
    > In the question they ask what is the first subnet of 10.0.0.0/19.
    > I thought it would be 10.0.0.0 with the first usable address as 10.0.0.1,
    > but they give the answer as 10.0.32.1 to 10.0.63.254.
    > Why isn't 10.0.0.1 to 10.0.31.254 the first subnet?


    For the same reason the first subnet is not used in the other two examples:

    192.168.214.0/29 -> First subnet 192.168.214.9

    172.28.0.0/20 -> First subnet 172.28.16.1

    so

    10.0.0.1/19 -> First subnet 10.0.32.1

    The problem is that applying the same, archaic, rule should also have
    excluded the LAST subnet from that list as well.

    This archaic rule had to do with the fact that the SubnetID portion of the
    address also could not be all zeros or all ones.

    Thus, 10.0.0.0/19 cannot be a SUBNET; 172.28.0.0/20 cannot be a SUBNET;
    192.168.214.0/29 cannot be a SUBNET.

    The question is severely misleading because it only partially applies the
    rule. If the rule is to be applied properly, then
    10.255.255.255/19 cannot be a subnet broadcast address, thus the subnet
    10.255.224.1/19 should be invalid.
    172.28.255.255/20 cannot be a subnet broadcast address; thus the subnet
    172.28.240.1/20 should be invalid.
    192.168.214.255/29 cannot be a subnet broadcast address; thus the subnet
    192.168.214.248/29 should be invalid

    But, as I noted, the rule is archaic. The explanation from the 70-291 book
    (p2-28) is much clearer:
    To determine the number of subnets available within an address space,
    simply calculate the value of 2^y, where y equals the number of bits in the
    subnet ID.... You do not usually need to subtract 2 from this total because
    most modern routers ... can accept a subnet ID made up of all 1s or all 0s.

    The 70-293 book has essentially the same statement, but because it did such
    a poor job of explaining the *subnet* related portion of this rule, the
    point is somewhat lost. From p2-27 of the 70-293 book:
    Most routers and operating systems, including Windows Server 2003, now
    enable you to use all zeros for a network or SUBNET identifier, but you must
    be sure that all your equipment supports these values before you decide to
    use them.


    The fact is that Microsoft TCP/IP has always allowed all 1s or all 0s; in
    fact, it was Microsoft TCP/IP that actually broke the rule to begin with and
    permitted it to be done on Windows NT, when other *standard* equipment was
    still conformant to the RFC that specified you could not do that.

    For purposes of Microsoft Certification Exams -- your answer, that
    10.0.0.1/19 is a valid subnet -- is correct.

    btw, note also there are several items of errata in that question, all of
    which are documented at
    http://support.microsoft.com/kb/832375/en-us




    --
    Lawrence Garvin, M.S., MCITP:EA, MCDBA
    Principal/CTO, Onsite Technology Solutions, Houston, Texas
    Microsoft MVP - Software Distribution (2005-2009)

    MS WSUS Website: http://www.microsoft.com/wsus
    My Websites: http://www.onsitechsolutions.com;
    http://wsusinfo.onsitechsolutions.com
    My MVP Profile: http://mvp.support.microsoft.com/profile/Lawrence.Garvin
    Lawrence Garvin [MVP], Jun 12, 2009
    #3
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